The modulation of the contractility of smooth muscle is crucial in the regulation of cardiovascular and respiratory physiology. Manipulation of smooth muscle contractility has utility in both the diagnosis and treatment of various conditions, notably hypertension and asthma. Although a number of receptors mediating the contraction of smooth muscle in response to neurotransmitters and hormones have been well characterized pharmacologically, studies of the probable biochemical basis of signal transduction (the phosphatidylinositol (PI) cycle) have only recently begun. The goal of this proposal are to confirm the PI response as the primary receptor-mediated event leading to smooth muscle contraction, and to elucidate the biochemical mechanisms of alterations in tissue responsiveness. Through a careful, kinetic study of stimulated and unstimulated labeling of the components of the PI cycle in rabbit aorta, the temporal sequence of events in the response to several agonists will be established. Known inhibitors and chemical modulators of components of the PI response and of contraction will be tested in both systems in order to establish a link between the biochemical and functional effects of these agonists. A physiological dissection of the contractile response into its tonic(slow) and phasic(fast) components by manipulation of Ca++ in the medium will aid in the detailed and specific elucidation of the biochemical mechanisms governing each. These experiments will serve as a prelude to experiments designed to probe the biochemical nature of modulations of tissue responsiveness. One such modulatory phenomenon, agonist-specific desensitization, will be correlated with corresponding phenomena in the PI response. Also studied will be the phenomenon of functional antagonism and the possibilities of receptor-receptor interactions. These will be studied by examining the effects of receptor and non-receptor mediated alterations in c-AMP metabolism on the contractile and PI responses.